1. Field of the Invention
This invention relates to roof bolts and more particularly to a roof bolt which is positioned in a bore hole drilled in a rock formation in a mine roof and which is held in place within the bore by a tensionable roof bolt system.
2. Description of the Prior Art
It is well established practice in underground mining work, such as coal mining, tunnel excavation, or the like, to reinforce or support the roof of the mine to prevent rock falls or cave-ins. One of the presently used systems to support a mine roof is an elongated bolt or bar which is inserted into a pre-drilled hole in the rock formation above the mine roof and which is securely fixed in the hole by an anchoring means such as a mechanical anchor. Other types of tensionable mine roof bolt systems are also known in the art. More recently the use of quick setting resins have been used in conjunction with mechanical anchors or the other tensionable systems to further support the roof bolt in the bore hole.
The mechanical anchor is generally a radially expanding expansion shell and an internally threaded tapered nut or spreader. The downward movement of the expansion shell is limited by a support mechanism such as a nut or the like. Once this mechanical anchor type of roof bolt is set with or without the resin system, the bolt is further tensioned.
More specifically, the bolt head or a nut, a washer and a bearing plate represent the portion of the anchoring system external to the pre-drilled bore hole. The system is placed in tension by tightening the bolt head or nut causing the head or nut, the washer and the bearing surface to be placed in compression This in turn also causes the mine roof which is within close proximity of the bearing surface to be placed in compression and thus provides improved roof support. It is therefore desirable to maximize the induced tension from the applied torque.
It has been recognized that a friction reducing washer positioned between the nut or bolt head and the bearing plate reduces the effects of friction and improves the level of tension in a given bolt as well as reducing the variability in tensioning from bolt to bolt. Most commonly used is a hardened steel washer which is recognized in the present A.S.T.M. specifications. Two other types of washers for this purpose are disclosed in U.S. Pat. Nos. 4,619,559 and 4,746,248.
The hardened steel washer does improve bolt tension/torque ratio to a point, but greater increases are desirable. A problem also exists with the hardened steel washer in that as the bolt is tensioned, the nut and washer tend to spark creating a potentially dangerous situation when methane gas is present. Further, the washer is subject to rusting which increases friction and reduces its effectiveness. The plastic washer disclosed in U.S. Pat. No. 4,619,559 does not generally withstand the applied forces and tends to fail by either ripping or extruding between the bearing plate and bolt head. The lubricated washer disclosed in U.S. Pat. No. 4,746,248 requires the application of heat generated by tightening the bolt to be effective and is thus variable from bolt assembly to bolt assembly. In addition, the latent lubricating material tends to squeeze out from the assembly under high boom pressures and further tends to collect dirt and other impurities which adhere to hot melt adhesives of type employed
It is, therefore, an object of the present invention to improve the mine bolt tension/torque relationship which is simple and inexpensive to implement.
It is yet another object of the present invention to improve the mine bolt tension/torque relationship which is not temperature dependent.